Rheumatoid factor (RF) is known to appear highly frequently in serum and synovial fluid of patients suffering from chronic articular rheumatism, one of the autoimmune diseases. It is also known that patients test positive for rheumatoid factor in collagen diseases, liver diseases, infectious diseases and other diseases as well. Assay for a rheumatoid factor is very useful in diagnosing and treating these diseases including chronic articular rheumatism.
Rheumatoid factor is considered an antibody formed by misrecognition of immunoglobulin. This is suggested, for example, by the fact that an immune complex comprising IgG and rheumatoid factor is formed in synovial fluid of chronic articular rheumatism patients. Traditionally, rheumatoid factor has been identified as a macroprotein with a molecular weight of about 1,000,000 belonging mainly to the IgM class. It has recently been confirmed, however, that there are other types of rheumatoid factor, belonging to the IgG class and IgA class. Assay for a rheumatoid factor by immunoglobulin class is expected to permit efficient elucidation of disease cause and efficient diagnosis.
The widely used assay methods are the RA assay method which utilizes latex agglutination and the RAHA assay method which utilizes passive hemoagglutination of sheep red blood cell. Although semi-quantitative determination by serial dilution is also available in these assay methods, all these methods are based on nothing more than qualitative reaction. Therefore, these assay methods give rise to difficulty, for example, in accurate determination of time-related changes in rheumatoid factor.
Some quantitative assay methods for a rheumatoid factor have been developed as improvement of these qualitative or semi-quantitative methods. Examples of the quantitative assay method for a rheumatoid factor include the laser nephelometric method, the turbidimetric immunoassay method and the labeling immunoassay method. Among these quantitative assay methods, neither the laser nephelometric method nor the turbidimetric immunoassay method permits accurate assay of rheumatoid factor of, for example, the IgG class, though they both permit assay of high molecular rheumatoid factor of the IgM class alone since they are based on agglutination. On the other hand, the labeling immunoassay method permits assay of rheumatoid factor by immunoglobulin class since it utilizes the high specificity and high detection sensitivity of antigen-antibody reaction wherein antigen or antibody is immobilized to an insoluble carrier. For example, the following method is described by Teitsson, I. and H. Valdimarsson in the Journal of Immunological Methods, 71, 149 (1984). First, an element is prepared which comprises an insoluble carrier such as polystyrene and immunoglobulin such as rabbit IgG or denatured human IgG coupled thereon. To this element is added a sample containing rheumatoid factor, and this is followed by rheumatoid factor binding to IgG in the element. The IgG-bound rheumatoid factor thus obtained is then reacted with each of labeled antibodies such as labeled anti-human IgG, labeled anti-human IgM and labeled anti-human IgA. The amount of each labeled antibody bound to the rheumatoid factor is then measured. This determination method permits assay of rheumatoid factor in the sample by immunoglobulin class (e.g. IgG-class RF, IgM-class RF or IgA-class RF). However, this method has a drawback that the immunoreactivity of carrier-coupled IgG decreases noticeably during storage in cases where the above-mentioned element comprising an insoluble carrier and IgG coupled thereto is prepared and stored in a dry state before use.
Since assay of rheumatoid factor by immunoglobulin class is very useful in etiologic elucidation, diagnosis and treatment of various diseases related to rheumatoid factor, it is expected that an assay element and a method will be developed which permit assay of rheumatoid factor with high reproductibility and high precision.